Nocardia, an organism closely related to Mycobacterium, can cause fatal infections in man. However, little is known about the mechanisms controlling nocardial invasion, infection, and disease production. In contrast, a great deal of work has been done on mycobacterial diseases. It has been shown that much of the organism's effect on the host as well as its virulence resides in the chemical make-up of the cell envelope. Therefore, I propose to study the in vivo and in vitro composition of the nocardial cell envelope with specific emphasis upon the in vivo action of various cell wall components. Conversely I will investigate the effect of the host as well as the environment on the chemical composition of the nocardial cell wall. I hope to determine how these components effect the host-parasite relationship. To initiate these studies I will describe the ultra-structural events that occur during infection by various nocardiae, while at the same time determining the chemical composition of the cell walls during growth both in vitro and in vivo. Ultimately the various cell wall components will be purified, characterized, and injected back into the host to determine at the ultrastructural and chemical levels what events occur with each fraction. The observed data will be compared to the actual disease process. By studying nocardial strains that vary from non-infectious to a fully virulent form I hope to characterize more completely the factors contributing to nocardial virulence.